1. Field of the Invention
The present invention relates to aqueous dispersions based on poly(meth) acrylates, produced in a multi-state emulsion polymerization process, and to the use of such dispersions to prepare adhesives (bonding compositions) which are susceptible to hot-sealing.
2. Description of the Background
In the past, hot-sealing compositions have been devised for the packaging industry, which compositions full- meet the applicable technical criteria, but which contain substantial amounts of organic solvents. E.g., Eur. Pat. 129.178 describes a hot-sealable coating composition comprised of a film-forming dispersion of at least two different polymer types with different adhesion properties in an organic solvent system, Wherewith the dispersion contains polymers of the two types and in addition contains a polymer comprised of units corresponding to the two polymer types. Vinylidene chloride polymers have also been used widely in the past as hot-sealable compositions.
Coating compositions for sealing metal substrates are of particular practical and economic importance; in particular, compositions for providing seals between aluminum and plastics such as polystyrene, polypropylene, and polyvinyl chloride, or between aluminum and glass. (See Ullmanns Enzyklopaedie der techn. Chemie, 6th Ed., 1985 ff., Vol 16, pub. Verlag Chemie, pp. 101-103.)
Ger. Pat 4,209,651 (to BASF AG) describes an aqueous dispersion containing a copolymer A in the presence of which a polymerization is carried out to produce a copolymer B. Copolymer A is produced by bulk polymerization, and is not a core-and-shell product. The product materials are used in sealing PVC to paper board.
Jap. Pat. 08-231729 A (Derwent No. 96-461419/46). The products comprise core-and-shell polymers produced by emulsion polymerization followed by spray drying. The shell has larger amounts of carboxyl-group-containing monomers which improve the re-dispersibility of the product in water.
Rising awareness of environmental issues has created a trend away from solvent-containing formulations and toward aqueous dispersions such that when. e.g. a coating or adhesive is applied to a surface, organic compounds are not emitted into the environment. Also significant is the trend away from chlorine-containing products, and, perhaps to a lesser extent, aromatic compounds.
Technology has been able to adjust, to some extent, to these more stringent requirements. Thus, as disclosed in Ger. 29 06 118, polymers of vinylidene chloride, used as hot sealing adhesives, may be applied in the form of aqueous dispersions. In order to improve adhesion to un-primed plastic and metal sheets, special xe2x80x9canchoring agentsxe2x80x9d may be added to the composition. Suitable such additives are water-soluble polymers of functionally substituted acrylamides and/or methacrylamides, or copolymers of these monomers with acrylamide and/or methacrylamide, which (co)polymers are present as species dissolved in the aqueous phase of the dispersion.
Two methods of coating of aluminum foils and sheets are in widespread use:
In the case of relatively aggressive filler materials, first a primer coat of about 2 microns in thickness is applied to the aluminum foil. This primer is comprised of a viniyl-chloride-containing solution polymer. A finish coating comprised of a methacrylate resin of 4-8 micron thickness is then provided over the primer coat.
For cases of less aggressive filler materials, a single-coat system is used, employing a hot-sealable coating of thickness of about 5-10 microns comprising a mixture of the abovementioned polymers.
Still unsolved is the problem of providing a non-chlorinated, hot-sealing adhesive in the form of an aqueous dispersion with adequate adhesion. Ger. 27 27 914 (U.S. Pat. No. 4,291,090) and Ger. 28 55 147 disclose hot-sealing adhesives based on polyacrylate which can be applied as aqueous dispersions. In some cases, these adhesives are not comprised exclusively of esters of (meth)acrylic acid. That is, they may contain one or more comonomers, employed, e.g., for the purpose of modifying the melting temperature, hardness, or adhesion properties of the adhesive. Suitable comonomers include, inter alia. (meth)acrylamide, and aminoalkyl esters of acrylic acid and/or of methacrylic acid. The specific effects which these comonomers have on the properties of the polymers is not disclosed.
Ger. 39 30 743 (Can. Pat. App. 2.025,368.1) describes an aqueous polyacrylate dispersion, which is useful as a hot-sealing adhesive, based on at least 70 wt. % of a polyacrylate comprised of units of lower alkyl methacrylates and also containing polymerized (meth)acrylamide or aminoalkyl (meth)acrylate. Other publications relate to aqueous dispersions of copolymers of olefins and/or vinyl acetate, which copolymers contain units of chlorinated monomers and are produced in a single-stage process. The described dispersions exhibit poor adhesion to aluminum substrates.
Polymer dispersions, which are prepared from polymer particles having a core-and-shell structure, which are described in the literature, either contain chlorinated monomers or are not suitable for bonding plastics to aluminum.
Eur. Pat. 574,803 describes aqueous polymer dispersions of 5-80 wt. % of a copolymer (A), having a glass transition temperature of 50-150xc2x0 C., and 95-20 wt. % of a copolymer (B), having a glass transition temperature of xe2x88x9250 to +50xc2x0 C., the two copolymers in the composition havingwith a temperature difference of 20xc2x0 C. There is no specific indication in this publication that the proposed products are suitable for bonding plastics to aluminum.
Further, no appropriate two-coat methods are described in the cited state of the art references.
A two-coat coating is proposed in Jap. Pat. App. 54-161,684 (Chem.Abstr. 92:199427x). First, an aluminum foil is coated with the aqueous dispersion of a monomer and then with a mixture of an ethylene-vinyl acetate copolymer. colophony, and wax, to a thickness of 20 microns.
In connection with the object of formulating chlorine-free aqueous polymer dispersions for use as hot-sealing adhesives, another problem exists which is the problem of compatibility with the state of the art apparatus employed. This problem is particularly significant in the large-scale coating of aluminum foils and sheets. In particular, one should be able to use customary methods of applying polymers as thin films (see Polymer Handbook, 2nd Ed., pub. Hanser Verlag), and the films employed should not be sticky at ordinary temperatures.
A requirement imposed on means of hot sealing an adhesive to a suitable substrate is that good adhesion for bonding the two materials (e.g. aluminum and polystyrene) must be achieved in a short time and at temperatures which are not excessive and which are about, e.g., 180-220xc2x0 C.
The proposed packaging methods are also intended for used with foodstuffs. In such applications, the monomers and the adjuvants must be selected such that the regulations governing food packaging (German Federal Health Office guidelines BGA 14, and the corresponding US FDA guidelines) are complied with.
In Ger. 39 30 743 an aqueous dispersion is proposed which has adequate adhesion to an aluminum foil substrate coated with a vinyl chloride copolymer as a primer. Efforts to modify such dispersions by incorporating units of adhesion-promoting monomers in order to provide a variant which can be applied to un-primed aluminum have not been successful. Thus the reference does not provide a solution to the problem of replacing chlorinated primers. Other efforts using similar approaches, using polymers with simple particle structures, have not been successful.
Accordingly, one object of the present invention is to provide an aqueous polymer dispersion of improved hot-sealing and adhesion characteristics to plastic and metal substrates.
Another object of the present invention is to provide aqueous poly(meth) acrylate dispersions PD, which are produced by emulsion polymerization and which have at least a two-stage particle structure, for use as hot-sealing adhesives.
Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by aqueous poly(meth)acrylate dispersions, comprising:
a particulate poly(meth)acrylate comprising the following components:
(A) 50-90 wt. % of methacrylic acid alkyl esters;
(B) 5-30 wt. % of an acrylic acid alkyl ester having a glass transition temperature Tg in the range c. xe2x88x9220 to 50xc2x0 C. and having formula I: 
wherein R1 represents an alkyl, aryl, or alkaryl group having 2-10 C atoms;
(C) 2-10 wt. % of a functionalized monomer of formula II: 
wherein R2 represents hydrogen or methyl; and R3 represents a glycidyl group 
or an alkyl group which has 2-6 C atoms and is substituted with at least one hydroxyl group;
(D) 2-10 wt. % of at least one radically polymerizable carboxylic acid or a polycarboxylic acid or a partially esterified polycarboxylic acid: and
(E) 0-40 wt. % of other monomers which are copolymerizable with (A) to (D); wherewith, in the process of preparation, a first polymerization stage (Process Step 1) is conducted with monomers of groups (A) to (C) and optionally (E), and in a second polymerization stage (Process Step 2), monomers of groups (A), (B), (D), and optionally (E) are copolymerized; and wherewith the sum of the amounts of monomers (A) to (E) is 100 wt. %.
The sum of the percentages of monomer units from groups (A) to (E) is 100 wt. %. The preferred monomer of formula I is n-butyl acrylate. Preferred monomers of formula II include glycidyl (meth)acrylate and hydroxyethyl (meth)acrylate.
Suitable monomers for group (E) monomers include styrene, (C1 to C4)-alkylstyrene compounds, cycloalkyl (meth)acrylates, and the like. Data on glass transition temperatures TG may be found in, e.g., Kirk and Othmer, 1978, xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, 3rd Ed., pub. J. Wiley, Vol. 1, pp. 387-389.
The ratio of the materials in the first polymerization stage to the materials in the second polymerization stage ranges from 30:70 to 70:30 parts by weight (pbw). A recommended rule for the proportioning of the polymerization stages is that the amounts should not differ by more than a factor of 2.5.
Additional preferred embodiments of the reaction include the following:
(i) The methacrylate content of the monomers polymerized in the first stage should be about 60xc2x120 wt. %.
(ii) The content of monomers of formula I in the monomers polymerized in the first stage should be about 30xc2x110 wt. %.
(iii) The content of monomers of formula II in the monomers polymerized in the first stage should be about 10xc2x15 wt. %.
(iv) The remainder of the monomers to complete 100 wt. % of the monomers in the first stage is the monomer of group (E).
Still further preferences include:
(i) The content of methyl methacrylate and/or monomers of group (E) in the monomers polymerized in the second stage should be c. 20xc2x110 wt. %.
(ii) The content of (meth)acrylic acid in the monomers polymerized in the second stage should be about 12xc2x18 wt. %.
For each stage of polymerization, the sum of the percentages of all the monomers is 100 wt. %.
The molecular weights of the poly(meth)acrylates in the dispersion PD are generally in the range  greater than 20,000 g/mol. The polymer content of the dispersions is generally in the range 20-65 wt. %; and the particle sizes are generally in the range 60-250 nm, preferably 80-160 nm, particularly preferably 100-140 nm (determined by photon correlation spectroscopy).
The poly(meth)acrylate dispersion PD of the invention is produced by emulsion polymerization in the aqueous phase in the presence of anionic, cationic, or nonionic emulsifiers, which emulsifiers are per se known, and in the presence of at least one radical-forming initiator, in a two-stage reaction (see Rauch-Puntigam, H., and Voelker, Th. 1967, xe2x80x9cAcryl- und Methacrylverbindungenxe2x80x9d, pub. Springer-Verlag, pp. 217-230).
It should be noted that certain advantageous properties, e.g. resistance to peeling, as specified in food packaging regulations (BGA 14 guidelines, and the corresponding US FDA guidelines), can be achieved with the use of additives.
The first stage may be conducted as a controlled-feed process: A suitable reaction vessel equipped with a stirrer and heater is initially charged with deionized water and emulsifier(s). Subsequently, after a certain temperature is reached, the initiator, preferably dissolved in water, is added. The initiator may be, e.g., an inorganic peroxide such as potassium peroxydisulfate (KPS) or ammonium peroxydisulfate (APS). It is recommended that the content of initiator in the mixture at this point be in the neighborhood of 0.8 wt. %, based on the weight of the monomers reacted in Process Step 1.
Suitable ionic emulsifiers, which are used in amounts of 0.01-2.0 wt. %, based on the total weight of monomers, include anionic emulsifiers Such as Aerosol OT75(R) of Cyaniamid BV and xe2x80x9cDowfax 2A1xe2x80x9d of Dow Europa SA.
The addition of the above-described mixture for Process Step 1 is a controlled feed over a prescribed period of time, e.g. 2 hr, in the first stage. As an example, the feed mixture for Process Step 1 may comprise 20-60 wt. % of the total water to be used, plus the monomers of groups (A) to (C) and optionally (E).
Advantageously, the mixture is then mixed for an additional period of time, e.g. 1 hr, at elevated temperature, e.g. 80xc2x0 C., following which it is allowed to cool, e.g. to 30xc2x0 C. The monomers described above in connection with xe2x80x9cProcess Step 2xe2x80x9d are then added dropwise over a relatively short period of time, e.g. 30 min, and the mixture is then allowed to stand for an appreciable period of time, e.g. 4 hr.
The resulting aqueous monomer/polymer mixture is then heated slightly, e.g. to 40xc2x0 C., and additional radical initiator, preferably a redox-type initiator, e.g. comprised of peroxydisulfate. dithionite, and iron(II) sulfate, is added in water, and further polymerization is initiated.
After the maximum temperature is reached, advantageously the temperature is controlled, e.g. at 80xc2x0 C., and the mixture is stirred for an additional period, e.g. 2 hr, to complete the polymerization.
Alternatively, the second stage can be carried out as another controlled-feed xe2x80x9csemicontinuousxe2x80x9d step, with addition of a monomer emulsion gradually over a period of, e.g., 2 hr.
The mixture is cooled to about 30xc2x0 C., and advantageously a suitable preservative is added, e.g. Acticid SPX(R) of Thor Chemie GmbH, along with a small amount of ammonia (as 25% ammonia in water).
One may also add 0-5% of nonionic emulsifiers, e.g. ethoxylated alcohols, or methacrylate esters of methoxypolyethylene glycols, e.g. xe2x80x9cCarbowax 550xe2x80x9d, or alkylphenols, and also an additional amount of an anionic emulsifier.
As a rule, the polymer dispersion (PD) may be used directly for coating. In certain cases limited amounts of thickeners or film-forming agents may be employed. The coating may be applied by spraying, brushing, pouring, dipping, blade-coating, or roll-coating. Generally the coating is applied in a thickness such that the dried coating has a thickness such that the dried coating layer is 2-10 microns thick.
Advantageously, the coated articles are dried in a drying oven or by continuously conveying the coated articles through a drying tunnel, possibly at reduced pressure and at temperatures in the range 100-200xc2x0 C.
In general, the higher the drying temperature, the shorter the drying time. Drying times are, e.g., in the range 5 sec to 5 min. It is possible to apply the coating in multiple coats. Preferably, the substrate to which the polymer dispersion (PD) is applied has a non-porous and continuous (xe2x80x9cclosedxe2x80x9d) surface, e.g. the surface of a plastic film or sheet, or particularly of a metal foil or sheet, e.g. aluminum foil or iron.
As stated above, one advantage of using the present poly(meth)acrylate dispersions (PD) as hot-sealing adhesives is that they solve the important and urgent problem of formulating a hot-sealable composition based on an aqueous acrylate dispersion. e.g. the type of dispersion disclosed in Ger. 39 30 74, which composition adheres strongly to substrates, particularly metal foils or sheets, and in particular aluminum foils or sheets.
Accordingly, the principal embodiment of the present invention is hot-sealable coated substrates, particularly of metal or plastic, coated with the present poly(meth)acrylate dispersion(s) (PD) as a primer layer, with a second layer applied over the primer layer comprised of an aqueous poly(meth)acrylate dispersion comprised of:
at least 70 wt. % of at least one C1-C4-alkyl ester of (meth)acrylic acid, and
an appreciable amount of (meth)acrylamide and/or aminoalkyl (meth)acrylate; wherewith the second layer is applied over the dried primer layer, and then dried.
For details beyond those provided in the example embodiments described below, reference is made to the disclosure of Ger. 39 30 743. A representative such dispersion and the means of its application are described hereinbelow (see Part II of the Example section).
In hot-sealing, the temperature in the coating must exceed the glass transition temperature of the poly(meth) acrylate. The temperature of the hot contact members which effect the sealing must exceed the required sealing temperature by an amount which increases as the contact time is decreased and as the heat conduction through the substrate layer is decreased. A thin metal foil has very good heat conduction, enabling the hot contact members to be at a temperature which is only slightly above the melting temperature of the poly(meth)acrylate. However, in practice the fastest possible sealing is desired, necessitating substantially higher temperatures of the hot contact members, e.g. 120-220xc2x0 C. In the case of a plastic substrate, the hot contact member temperature may have to be limited because of the glass transition temperature of the plastic. To obtain a seal of high strength, the pressure exerted by the hot contact members should be at least 1 kp/cm2, preferably 3-6 kp/cm2.
The effective peeling resistance of hot-sealing samples from the Examples seas tested in the customary fashion (sealing seam strength according to DIN 51 221).
The objective was to demonstrate that, e.g., the use of the present poly(meth)acrylate dispersions as primers on aluminum foils or sheets, along with the use of the aqueous dispersions of Ger. 39 30 743, as an outer hot-sealable layer provides adhesive properties equivalent to those provided with primers based on solvent-containing and/or chlorinated polymers.